Chemical-mechanical polishing (“CMP”) processes remove material from the surface of semiconductor wafers or other substrates in the production of microelectronic devices and other products. FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a planarizing liquid 44 on the polishing pad 40. The polishing pad 40 and the planarizing liquid 44 may separately, or in combination, define a polishing medium that mechanically and/or chemically removes material from the surface of a wafer. The polishing pad 40 may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be a new generation abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid 44 may be a conventional CMP slurry with abrasive particles and chemicals that is used with a conventional polishing pad, or the planarizing liquid 44 may be a planarizing solution without atbrasive particles that is used with an abrasive polishing pad.
The CMP machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the polishing pad 40. A drive assembly 26 rotates the platen 20 (indicated by arrow A), or it reciprocates the platen 20 back and forth (indicated by arrow B). Since the polishing pad 40 is attached to the under-pad 25, the polishing pad 40 moves with the platen 20 during planarization.
The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier; or an actuator assembly 36 may be attached to the wafer carrier to impart axial and/or rotational motion to the wafer 12 (indicated by arrows C and D, respectively).
To planarize the wafer 12 with the CMP machine 10, the wafer carrier 30 presses the wafer 12 face-downward against the polishing medium. More specifically, the wafer carrier 30 generally presses the wafer 12 against the planarizing liquid 44 on the planarizing surface 42 of the polishing pad 40, and at least one of the platen 20 or the wafer carrier 30 moves relative to the other to move the wafer 12 across the planarizing surface 42. As the wafer 12 moves across the planarizing surface 42, material is removed from the face of the wafer 12.
In the competitive semiconductor industry, it is desirable to consistently stop CMP processing of a run of wafers at a desired endpoint and to produce a uniform, planar surface on each wafer. Accurately stopping CMP processing at a desired endpoint is important to maintaining a high throughput of planarized wafers because the planarized surface must be at a desired level with respect to other layers of material and structures on the wafer. For example, if the planarized surface is above an acceptable level, the wafer must be re-planarized until it reaches a desired endpoint. Additionally, it is important to accurately produce a uniform, planar surface on each wafer to enable precise circuit and device patterns to be formed with photolithography techniques. The critical dimensions of many photo-patterns must be focused within a tolerance of approximately 0.1 μm. Focusing photo-patterns to such small tolerances, however, is difficult when the planarized surface of the wafer is not uniformly planar. Therefore, two primary objectives of CMP processing are stopping planarizing at a desired endpoint and producing a highly uniform, planar surface on each wafer.
The endpoint of CMP processing may be determined by estimating the time-to-polish the wafer based on the polishing rate of previous wafers. CMP processing, however, involves many operating parameters that affect the planarity of the surface of the wafer and the ability to estimate the time-to-polish a wafer to a desired endpoint. The rate at which the material is removed from the surface of the wafer (the “polishing rate”) often varies from one wafer to another. The most common parameters that affect the polishing rate of a wafer are: (1) the relative velocity created between the wafer and the polishing pad across the face of the wafer; (2) the distribution of slurry across the surface of the wafer; (3) the composition of materials of the wafer; (4) the topography of the wafer; (5) the parallelism between the face of the wafer and the surface of the polishing pad; (6) the temperature gradient across the face of the wafer; and (7) the condition of the planarizing surface of the polishing pad. The polishing rate may vary from one wafer to another because it is difficult to identify and correct changes in specific operating parameters. Thus, it is difficult to consistently stop CMP processing at a desired endpoint on a wafer by estimating the time-to-polish the wafer using the polishing rate of previous wafers.
The endpoint of a wafer may also be determined by stopping CMP processing and measuring a change in thickness of the wafer. In a typical process for measuring a change in thickness of the wafer, the wafer is partially or completely removed from the planarizing surface of the polishing pad, and then an interferometer or other measuring device measures a change in thickness of the wafer. However, repeatedly stopping CMP processing to measure the change in thickness of the wafer reduces the throughput of planarized wafers, or a wafer may be destroyed or impaired because it may be over-polished beyond an acceptable endpoint before the first measurement. Accordingly, it is also difficult and time-consuming to consistently stop CMP processing at a desired endpoint by continuously measuring the actual change in thickness of the water.
In light of the problems with determining the endpoint of CMP processing, it would be desirable to develop a method and apparatus that indicates when a wafer has been planarized to a desired endpoint.